This invention relates to a digitally controlled active phase shifter and more particularly to a digitally controlled phase shifter employing components which can be formed by microwave monolithic integrated circuit (MMIC) techniques.
The introduction of phased array antennas with the capability of beam steering and multiple beams has provided a major application for phase shifters and time delay networks. The ideal phase shifter shifts a phase of an applied signal the same amount at all frequencies within a wide band. Phase shifters have been widely employed in the prior art. Such prior art phase shifters have utilized diode switching devices, such as PIN diodes and other switching devices as well. Such switched phase shifters are designated as switched line phase shifters, lowpass/highpass phase shifters, loaded-line phase shifters and circular or hybrid coupler reflection phase shifters.
See a text entitled Microwave Semiconductor Circuit Design by W. Allen Davis, published by Van Nostrand Reinhold Co. (1984) pages 264-293 chapter 13 entitled "Circuits Using Pin Diodes". As one can ascertain, the phase shifter is normally employed in a transmit/receive (T/R) module for uses in phased array antenna systems and in EW applications. As one will understand, active phase shifters are known in the art and these are the type of phase shifters that provide gain instead of loss. Lossy phase shifters incorporate passive devices. The size of an active phase shifter is much smaller than a passive phase shifter, and hence such active phase shifters are more desirable in regard to modern day technology.
As indicated above, such phase shifters are widely employed and particularly attractive in applications involving phased array antenna systems. Such antennas normally consist of an assembly of closely packed radiator elements which are driven by a microwave transmit/receive (T/R) circuit module. In a phased array radar the radiating elements are fed varying amplitude and phase signals to shape and position the beam. Passive phase shifters as indicated have been used in such arrays. The passive phase shifters are large in size even when used in hybrid MIC formats. There are difficulties involved with such phase shifters as above indicated.
For other pertinent background reference is made to a copending application Ser. No. 255,082 entitled BIDIRECTIONAL DIGITAL PHASE SHIFTER APPARATUS filed on Oct. 7, 1988 for T. Chen and M. Kumar, the inventors herein and assigned to Siemens Corporation, the assignee herein.
As one can ascertain from the above copending application, there is described a bidirectional phase shifter which can be implemented in monolithic format (MMIC). The phase shifter of the application operates in broad band operation with excellent amplitude and phase tracking. The phase shifter is a bidirectional phase shifter which employs constant resistance all pass LC networks which networks operate to pass wide bands of frequencies with any desired phase shift within the band. Such networks provide a constant phase delay while having superior amplitude characteristics for a wide band of input signals while avoiding many of the prior art problems. The copending application shows such all pass LC networks operating in conjunction with bidirectional single pole double throw (SPDT) switches fabricated from FET devices.
As also indicated in the copending application, the prior art has attempted to provide phase shifting apparatus utilizing active semiconductor devices such as MESFETS fabricated on gallium arsenide substrates employing MMIC technology. For an example of such prior art approaches, reference is made to U.S. Pat. No. 4,599,585 issued on July 8, 1986 and entitled N-BIT DIGITALLY CONTROLLED PHASE SHIFTER to J.L. Vorhaus et al. This patent discloses an N-bit digitally controlled phase shifter for controlling the phase of an applied signal in a range of zero to 360 degrees. As one can ascertain, the patent utilizes field effect transistors and transmission lines. Phase shift and incremental phase delay is obtained by switching the transistors to increase or decrease the effective length of the transmission lines.
Reference is also made to U.S. Pat. No. 4,458,2119 entitled VARIABLE PHASE SHIFTER issued on July 3, 1984 to J.L. Vorhaus. This patent also shows a phase shifter which includes three cascade interconnected phase shift stages. Each stage includes a quadrature coupler and a pair of field effect transistors in a common source configuration. The patent discloses circuits which are employed by cascading various substages to obtain variable phase shifting utilizing the FET devices to select the signal paths through the required stages.
Reference is also made to U.S. Pat. No. 4,549,152 entitled BROAD BAND ADJUSTABLE PHASE MODULATION CIRCUIT issued on Oct. 25, 1985 to M. Kumar, one of the inventors herein and assigned to the RCA corporation. This patent shows a phase modulation circuit to provide at an output terminal an output signal which is controllably shifted from 0 to 360 degrees relative to an input signal. The patent shows the use of various amplifier stages which are employed to provide selective phase shifts and which are operated with gain to control the amount of phase shift through the circuit.
The above-noted patents apart from showing various active devices in different types of phase shifting networks show integrated circuit techniques which can be employed to implement phase shifters for microwave application.
For further examples of phase shifters employed for microwave operation, reference is made to an article entitled "Monolithic Dual Gate GaAs FET Digital Phase Shifter" by J.L. Vorhaus et al., published in IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-30 No. 7, July 1982, pages 982 to 991. This article also describes monolithic microwave integrated circuit techniques utilized to provide various phase shifting devices.
Reference is also made to an article entitled "The Monolithic Single Chip X-Band 4-Bit Phase Shifter" by Y. Ayasli et al., published in the IEEE Transactions on Microwave Theory and Techniques. Vol. MTT-30 No. 12, December 1982, pages 20-201 to 20-205. This article shows the use of FET's as microwave switches employed with passive phase shifters which are capable of providing phase shift bits in predetermined increments. The article shows cascading of various stages to produce multi-bit digital phase shifters with low insertion losses which can be fabricated utilizing MMIC techniques on gallium arsenide substrates.
As one will understand when reviewing the above articles and patents, many such phase shifters employ gallium arsenide FET's or MESFET devices which are utilized as switches to essentially operate in conjunction with transmission lines, lowpass/highpass circuits, loaded lines or circulators and so on to produce variable phase shift characteristics of relatively wide bandwidths.
As indicated above, such phase shifters are subject to many problems in regard to operation, including input/output mismatch as well as nonlinear amplitude and phase shifting over relatively broad ranges.
It is a main object of the present invention to provide a digitally controlled phase shifter which operates with constant amplitude and excellent phase characteristics over a relatively wide bandwidth.
The subject matter phase shifter incorporates circuit components which are capable of being integrated using MMIC techniques on gallium arsenide substrates.
The digitally controlled monolithic active phase shifter according to the present invention utilizes active devices and cascode circuit configurations with common gate FET's employed as switching means. The cascode configuration as employed provides high isolation and excellent phase and amplitude stability.
The variation of the input impedance of the power dividers incorporated in the present invention is relatively small for all possible combinations and hence the circuit provides small incident phase changes. The circuit also employs a vernier control used with a power combiner and hence enables the capability of adjusting each phase bit to extremely fine accuracy. Due to the nature of the circuit components employed, the circuit is broad band and of relatively small size.
The circuit employs digital control signals which control signals are required, for example, in a phased array radar system to change the direction and the shape of the beam. The circuit can be easily implemented in integrated circuit form while employing components which are capable of being analyzed utilizing computer modeling and hence allowing easier design and circuit operation analysis.